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SubjectRe: [patch 13/14] x86_64: Use common functions in cluster and physflat mode
On Mon, Sep 12, 2005 at 01:02:20AM +0200, Andi Kleen wrote:
> On Sun, Sep 11, 2005 at 09:44:16AM -0700, Zwane Mwaikambo wrote:
> > On Fri, 9 Sep 2005, Ashok Raj wrote:
> >
> > > IPI performance worse, since it would do one cpu at a time, and requires 2
> > > writes per cpu for each IPI v.s just 2 for a flat mode mask version of the API.
> >
> > I don't see the benefit then :/ I certainly hope we don't go that route.
>
> I originally made that objection, but Ashok then did some benchmarks
> that showed essentially no difference. I can see the point - it's


Just a minor difference is that for a 8 CPU system

Using IPI shortcut uses just 1 write to local apic
Using mask version of broadcast uses 2 writes to local apic

The stat showed no significant difference when we do the 1 extra write. But
that is for the whole system.

When we use send_IPI_mask_sequence() we use 14 writes to (two writes
for each CPU that we need to target a IPI).

Using the same stats i used earlier, i see about 0x200 - 0x1000 extra cycles
when using mask_sequence() on certain long runs about 100K samples.

We should probably remove the !HOTPLUG case and just use the mask version
for all cases <=8 CPUS, use physflat or the cluster mode for >8cpus as
the case may be, instead of defaulting to sequence_IPI which seems
a little overkill for the intended purpose.

> likely that an access to the local APIC (which is in the CPU) is fast
> (we know it is) and all the time is dominated by sending the
> requests over the wires between the CPUs. So it shouldn't matter
> much if you use sequence mode or masks.
>
> That is why I changed my mind and just made physflat default for the hotplug
> case (which will be essentially everywhere because I expect most kernels
> to have hotplug enabled in the future)
>
> It's a bit of a mess in mm right now because me and Ashok have been fixing
> similar problems in a different way (e.g. the patch in flat to
> use the sequence sending path is also not needed anymore with that)
> Need to clean this up a bit.
>
> Handling it properly for i386 is also still needed e.g. the older physflat
> patch I did needs to be merged with bigsmp and cleaned up a bit.
> But I don't have time to do it before monday so it'll miss the 2.6.14
> window anyways.
>
> -Andi

--
Cheers,
Ashok Raj
- Open Source Technology Center
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